1. Field of the Invention
The present invention relates to a color cathode ray tube having a shadow mask and, particular to a color cathode ray tube having a face plate, which prevents deterioration of an image caused by thermal deformation of the shadow mask.
2. Description of the Related Art
In general, a color cathode ray tube having a shadow mask comprises an envelop having a face plate and a funnel jointed to the panel. The face plate has a substantially rectangular effective area, which is formed of a curved surface, and a skirt portion provided on an outer peripheral portion of the effective area, and the funnel is jointed to the skirt portion. Formed on the inner surface of the effective area of the panel is a phosphor screen which is formed of three-color phosphor layers for emitting three colors, i.e., blue, green, and red. In the envelope, a shadow mask is arranged to face the phosphor screen. The shadow mask has a mask body having a large number of electron beam apertures, and the mask body is formed in the shape of a curved surface.
In a neck portion of the funnel is arranged an electron gun for emitting three electron beams. Three electron beams emitted from the electron gun are deflected by the magnetic field generated by a deflection yoke, which is mounted on the outside of the funnel, and horizontally and vertically scan the phosphor screen through the shadow mask. Thereby, a color image is displayed on the screen.
According to the above-structured color cathode ray tube, in order to display a color image having good color purity on the phosphor screen, it is needed that the three-color phosphor layers and the shadow mask are correctly arranged to have the relationship of a predetermined matching such that the three-electron beams, which pass through each electron beam aperture of the shadow mask and enter the phosphor screen, land on the corresponding phosphor layers, respectively. For this purpose, it is important to set the distance (value q) between the inner surface of the panel and the shadow mask to a design value.
However, even if the three-color phosphor layers and the shadow mask are arranged to have the predetermined positional relationship, deterioration of color purity still occurs due to thermal deformation of the shadow mask in the color cathode ray tube. Specifically, the area in which the electron beam apertures are formed accounts for to 1/3 or less of the entire mask body, and the most part of the electron beams collides with the shadow mask, and thus, the shadow mask is heated. Generally, a mask body is formed of a low carbon steel plate having iron as a main ingredient and thermally expands by the above-mentioned heating toward the phosphor screen. This expansion of the shadow mask is so called as doming. As a result, the value q varies, and the landing position of the electron beams onto the three-color phosphor layers shifts from a desired position, thereby deteriorating color purity.
The shift of the landing position (mislanding) of the electron beams onto the three-color phosphor layers due to the thermal expansion of the shadow mask differs depending on an image pattern, which is radiated on the phosphor screen, and radiating time of the image pattern.
More specifically, if an image is radiated on the phosphor screen for a long time, not only the mask body having a large number of electron beam apertures but also a mask frame, which is attached to the peripheral portion of the mask body and has a large thermal capacity, are heated. However, as disclosed in Published Examined Japanese Patent Application No. 44-3547, such a mislanding due to the heating can be effectively compensated by attaching an elastic support member supporting the shadow mask to the mask frame through bimetal. On the other hand, as mislanding, which occurs for a short period of time, there is a local mislanding which is generated when an image having high luminance is locally radiated on the screen. Such a mislanding cannot be compensated by the compensating means, i.e., bimetal.
In other words, if an image having high luminance is locally radiated on the phosphor screen by a high-current beam, local doming is generated in the mask body by collision of the high-current beam. In the doming part of the mask body, the electron beam apertures shift from the normal positions to the other positions. Due to this, the electron beams, which pass through the electron beam apertures formed at the normal positions and correctly land on the three-color phosphor layers, cannot land on the normal positions of the three-color phosphor layers since the electron beams pass through the electron beam apertures displaced at the other positions. Such a local mislanding cannot be compensated by compensating, i.e., bimetal.
In order to examine the relationship between the high-current beam pattern and the mislanding which occurs for a short period of time, electron beams having a rectangular pattern were radiated on a phosphor screen through a shadow mask by means of a signal generator, and the shape, the size, and the landing position of the rectangular pattern onto the shadow mask were variously changed. As a result, it was ascertained that the amount of the mislanding, i.e., the distance between the actual landing position of the beam and the correct landing position thereof, was relatively small when the high-current beam pattern was radiated over substantially the entire surface of the phosphor screen. However, when the high-current beam pattern, which is elongated in a vertical direction, was radiated on that portion of the screen which is slightly apart from the peripheral portion of the screen toward the center thereof in the horizontal direction (X axis direction), the amount of the mislanding becomes the largest.
The relationship between the two types of high-current beam patterns and the mislanding can be explained as follows:
Generally, a television cathode ray tube is designed such that current to be supplied does not exceed a constant value which corresponds to an average cathode current of the cathode ray tube. In a case that the high-current beam pattern is radiated over substantially the entire surface of the phosphor screen, therefore, a current, which flows into the shadow mask per unit area, is smaller than the case that a high-current beam pattern with a small size is radiated. Thus, the rise in temperature of the shadow mask is small. Moreover, in the case that the high-current pattern with a small size is radiated on the central portion of the phosphor screen, mislanding hardly occurs even if the shadow mask is thermally deformed. However, as the beam pattern is moved from the central portion of the phosphor screen to the horizontal peripheral portion thereof, the frequency of the thermal deformation of the shadow mask, which appears on the screen as a mislanding, becomes high. However, in the vicinity of the horizontal peripheral portion of the phosphor screen, since the peripheral portion of the mask body is attached to the mask frame, the amount of the deformation of the mask body is small. Consequently, at that portion of the mask body which is slightly apart from the horizontal peripheral portion of the mask to the central portion thereof, the amount of the thermal deformation of the shadow mask is large and the amount of the mislanding becomes the largest.
Particularly, in a recent color cathode ray tube, an FS (Flat Square) tube in which an effective area of the face plate is flattened is mainly used. In this type of the color cathode ray tube, the mask body is flattened to correspond to the effective area of the panel. Therefore, in such a color cathode ray tube, mislanding of electron beams due to thermal deformation of the shadow mask increases.
Published Unexamined Japanese Patent Applications No. 61-163539 and No. 61-88427 disclose structures for compensating the mislanding of electron beams, in a color cathode ray tube whose effective area of the face plate is flattened, by improving the shape of the shadow mask. However, in a cathode ray tube having a flattened effective area, it is impossible to sufficiently compensate the mislanding of the electron beams only by changing the shape of the shadow mask.
Published Unexamined Japanese Patent Applications No. 64-17360 and No. 1-154443 disclose a structure wherein the mislanding is compensated by changing the shape of the effective area of the face plate together with the shadow mask. However, even if such a compensation is made, sufficient correction cannot be obtained in color cathode ray tubes, which have been recently developed, having a face plate which includes a substantially spherical effective surface such that an external image reflecting on the outer surface of the face plate is natural without making a user feel visually uncomfortable.